Argumentum ad hominem (from the Latin, "argument to the person") is an informal logical fallacy that occurs when someone attempts to refute an argument by attacking the claim-maker, rather than engaging in an argument or factual refutation of the claim. There are many subsets of ad hominem, all of them attacking the source of the claim rather than attacking the claim or attempting to counter arguments. They are a type of fallacy of relevance.

The fallacy is a subset of the genetic fallacy, as it focuses on the source of the argument, at the expense of focusing on the truth or falsity of the actual argument itself.

An ad hominem should not be confused with an insult, which admittedly attacks a person, but does not seek to rebut that person's arguments by doing so — that type of rhetoric is better termed as poisoning the well.

Of note: if the subject of discussion is whether or not somebody is credible and/or competent — e.g., "believe X because I am Y" — then it is not an ad hominem to criticize their qualifications.

While an ad hominem attack is not synonymous with "crass insult" (see below) it is also true that you can make a fallacious ad hom argument without being rude or crass about it. As a result, these can go unacknowledged as fallacious.

The "circumstantial ad hominem", or "appeal to motive", happens where an opponent's argument is discarded on the basis that they have some motivation for making it; for example, that it is in a banker's best interests to say he has not stolen from his company's accounts, so obviously he has. All "well they would say that, wouldn't they?" arguments are based on this form of ad hominem and can regularly be found propping up conspiracy theories when their existence is denied by an authority. A good rule of thumb to spot the fallacy here is that this sort of argument devalues the denial, but does not bolster the original assertion.

A personal attack can also be a cause for aesthetic judgement, such as in condemning a creative work created by an infamous person. Examples include Charles Manson's status as a multiple murderer and cult leader would affect reviews of his song-writing; similarly with Jimmy Savile and Gary Glitter'ssexual assaults tainting their previous work; and also regarding Richard Wagner's well-known anti-Semitism and later associations with the Third Reich, although this hasn't deterred notable Jewish Wagner enthusiasts including Gustav Mahler and Stephen Fry. "Separating the art from the artist", therefore, has become a core area of debate in literary and artistic criticism.

Often, ad hominem attacks are used subtly in order to influence the views of spectators. This is often termed poisoning the well where it occurs before an argument has been made, and is a form of psychologicalpriming. One could point out bad things that the opponent has done in the past, for instance, or establish an untrustworthy track record. In arguments about morality, one could make the argument that the opponent does not practice what they preach. This is a special case called ad hominem tu quoque.

Ad hominem arguments can work to convince people via a combination of the halo effect and cognitive ease. The halo effect is a cognitive bias in which the perception of one trait is influenced by the perception of an unrelated trait, e.g. treating an attractive person as more intelligent or more honest. Thus, if you can attribute a bad trait to your opponent, others will tend to doubt the quality of their arguments. With respect to cognitive ease, by repeating the ad hominem enough times, the cognitive strain required to reject someone's argument is lowered. By sufficient association with negative personal traits, rejecting an argument (with or without thorough evaluation) becomes a favoured option.

As ad hominem arguments are only fallacious if they do not follow (non sequitur)-if the argument and the person's character are related then there may not be a fallacy. In particular, a criticism is not an ad hominem argument if a person's merits are actually the topic of the argument. If the subject of the debate is the inherent trustworthiness of someone, or what prior probability you would assign to them telling the truth, then their previous track record is relevant to the subject. If debating a person's ability to do a task, then their effectiveness at that task or suitably similar ones, is relevant.

Ad hominem attacks are strictly fallacious when the attack has little or no bearing on the argument at hand. For example, dismissing a female scientist's opinion on a subject because she is a woman would be a fallacious ad hominem argument. Dismissing it for being incorrect with relevant evidence or reasoning, but making a sexist comment at the same time, would not employ formally fallacious reasoning. On the flip-side, dismissing that evidence or reasoning because it came packaged with a sexist comment wouldn't follow.

Of course, the fact that something is not a logical fallacy does not automatically make it a desirable debating tactic. Even if the underlying argument is sound, snide remarks and obnoxious insults can form an effective part of the rhetoric and cause people to dismiss an argument as the incidental attack triggers their own prior biases (see above).

Strangely, the people who cry about "ad hominem" the most tend to be those who makead hominems the most. This may be because the people who tend to care about ad hominem attacks (since they, unlike most people, aren't able to brush them off and get back to the substance of the debate) are also those who aren't able to make more substantive attacks than "You're dumb."

Alternatively, people all too often cry "ad hominem" when their debate opponent insults them, while failing to see the opposing arguments.[3]

The traditional meaning of the phrase is that an "argumentum ad hominem" is one tailored to appeal to the person with whom one is arguing rather than to impartial reason. This definition may be seen in Modern English Usage by H. H. Fowler,[4] a book whose explanations of usage are often taken as highly authoritative, if not definitive. For example, if you wanted to convince someone that Costa Rica was a superior place to go on vacation and you knew that the person was a keen birdwatcher, you might point out that Costa Rica is full of interesting bird life.